Quasi-experimental evidence is needed on the relations between human health and airborne particulate matter. April 22nd is the 45th Earth Day, which marks the birth of the modern environmental movement that helped lead to the creation of the U.S. Environmental Protection Agency, the Clean Air Act Amendments, and the Clean Water Act. The result has been substantial improvements in environmental quality in the United States. Today, developing countries are contending with levels of pollution that are even higher than those in the United States before the first Earth Day. And in a period of considerable economic difficulty, the United States is trying to strike the right balance between the benefits and costs of further reductions in pollution.